I have this same problem, the noise is like a droning noise, right from pressing the toilet flush button until the cistern is full. It can be very loud like a foghorn or quieter and more rumbly. If you turn on the cold tap the noise stops but as soon as you turn if off the noise starts again. Please help its doing my nut in!!

You have a loose washer. Either in the toilet fill valve or in the stop valve under the tank (cistern).

Which type of valve do you have?

Hi Paulajc and welcome to The Plumbing Page on AMHD.
You piggybacked on a 1 year old dead thread so I gave you one of your own. Before you post look in the upper left hand corner for the date..
Please answer Bobs question about which ballcock you have. My bet's on door #1.
Noise is caused by vibration. Something loose some where. But before we can help we have to have more details. Regards, Tom